Nipped rolls are used in a vast number of continuous process industries including papermaking, steel making, plastics calendering and printing. The characteristics of nipped rolls are particularly important in papermaking. In the process of papermaking, many stages are required to transform headbox stock into paper. The initial stage is the deposition of the headbox stock onto paper machine clothing or felt. Upon deposition, the white water forming a part of the stock flows through the interstices of the felt, leaving a mixture of water and fiber thereon. The felt then supports the mixture, leading it through several dewatering stages such that only a fibrous web or matt is left thereon.
One of the stages of dewatering takes place in the press section of the papermaking process. In the press section, two or more cooperating rolls press the fiberous web as it travels on the felt between the rolls. The rolls, in exerting a great force on the felt, cause the web traveling thereon to become flattened, thereby achieving a damp fibrous matt. The damp matt is then led through several other dewatering stages.
The amount of nip pressure applied to the web is important in achieving uniform sheet characteristics. Variations in nip pressure can affect sheet moisture content and sheet properties. Excessive pressure can cause crushing of fibers as well as holes in the resulting paper product. Conventional methods addressing this problem have been inadequate and, thus, this problem persists in the press section, often resulting in paper of poor quality, having uneven surface characteristics.
Roll deflection, commonly due to sag or nip loading, is a source of uneven pressure distribution. Worn roll covers may also introduce pressure variations. Rolls have been developed which monitor and alter the roll crown to compensate for such deflection. Such rolls usually have a floating shell which surrounds a stationary core. Underneath the floating shell are pressure regulators which detect pressure differentials and provide increased pressure to the floating shell when necessary.
Notwithstanding the problem of roll deflection, the problem of uneven loading across the roll length and in the cross machine direction persists because pressure is often unevenly applied along the roll. For example, if roll loading in a roll is set to 200 pounds per inch, it may actually be 300 pounds per inch at the edges and 100 pounds per inch at the center.
Methods have been used to discover discrepancies in applied pressure. One such method known as taking a nip impression requires stopping the roll and placing a long piece of carbon paper, foil, or impressionable film in the nip. One must load the rolls carefully to ensure that both sides, that being front and back, are loaded evenly. The pressure in the nip transfers a carbon impression, deforms the foil, or ruptures ink containing capsules in the film, indicating the width of contact. These methods for taking a nip impression are not reusable as they determine only a single event such as the highest pressure or contact width.
One of the major difficulties in using the nip impression procedure is that of evenly loading the rolls from front to back. The goal of the procedure is to measure and record the final stable loading along the length of the rolls after the initial loading. Often, during the initial loading, however, one end will contact before the other end. Thus, there are times when one end is heavily loaded while the other end is only slightly loaded. When this occurs, the nip impression shows the highly loaded condition and not the final state because the carbon paper, foils, and Prescale films record the largest width and/or highest pressures.
Another method of determining the nip pressure profile is to use a Prescale film that measures pressure. The film is fed into the nip after the rolls are loaded. Therefore, the film records the stable loaded condition rather than the greatest consequence of the loading process. Such a process eliminates the loading difficulties associated with nip impressions. Nonetheless, the Prescale films must be interpreted using a densitometer. This process is cumbersome, time consuming, and generally inefficient. Furthermore, the Prescale films are not reusable. A new piece of film must be fed into the nip after any corrective adjustments are made. Lastly, the Prescale films are temperature and moisture dependent, thus leading to inaccurate and unreliable results.
After a successful nip impression is taken, the carbon paper, foils, and Prescale films are removed from the nip and examined. Typically, the nip width is measured at twenty-one locations across the machine. These readings should be accurate to the nearest 1/64″ or 0.01″ for accurate interpretation. These measurements are time consuming and are subject to operator variations. Also, if the measurements require a change in the nip settings, a new piece of carbon paper foil, or Prescale film must again be placed in the nip. A common practice is to postpone performance of a confirming test until the next available shutdown. Thus, the processing may continue at a less than optimal state.
Crown corrections are often made from nip width measurements. For simple crown corrections, the amount of correction may be estimated from:
  C  =            (                        N          E          2                -                  N          C          2                    )        ⁢                            D          1                +                  D          2                            2        ⁢                  D          1                ⁢                  D          2                    where                NE is the nip width at the end of the roll,        NC is the nip width at the center of the roll, and        D1 and D2 are the roll diameters.This equation is used throughout the paper industry for estimating crown corrections.        
Various methods have been used to alleviate some of the challenges discussed previously. In U.S. Pat. No. 3,906,800 to Thettu, a reusable nip measuring device and method are disclosed. This method uses two polyamide sheets, one of which is coated with silicone rubber. When placed in the nip and when the nip is closed, the two sheets contact and fuse within the contact region. The nip is reopened, the sheets are removed, and the nip width distribution may be measured in a manner similar to carbon paper, foils, and Prescale films. Thus, the interpretation times are not improved. This method has the advantage of reusability, but is subject to the same loading path challenges in that it will record the greatest contact width and not necessarily the final stable state.
In U.S. Pat. No. 4,744,253 to Hermkens, a system is disclosed that uses ultrasonic waves to determine the thickness of a thin film sensor. The time difference between the transmission pulse and the received pulse is related to the pressure on the sensor. This time difference is used to measure the sensor cladding thickness, which is related to the applied pressure. Because this technique does not provide nip width measurements, it is inconvenient to use to make crown corrections.
In U.S. Pat. No. 4,016,756 to Kunkle, a nip load sensing device is disclosed. This device uses a bar containing load cells that are placed in the nip. The method provides discrete load reading across the machine, but does not produce nip width measurements. Thus, crown corrections do not follow directly.
U.S. Pat. No. 5,379,652 to Allonen discloses a method and device for measuring nip force and/or nip pressure in a nip. The Allonen system measures nip pressure, rather than nip width, and uses information gathered during the measurement of nip pressure to estimate nip width. Ordinarily, the piezoelectric sensors employed by Allonen require a dynamic event (such as passing through a nip) in order to operate because such sensors measure changes in pressure and vary the signal based on the amount of pressure.
U.S. Pat. No. 5,383,371 to Laitenen discloses a method and device for measurement of nip force and/or nip pressure in a nip formed by a revolving roll or a band used in the manufacture of paper. This device also employs force or pressure detectors formed of piezoelectric film. The device may be used to estimate nip width, but similarly requires a dynamic event, namely, that the measurements be made as the sensors are passed through the press nip.
U.S. Pat. No. 5,048,353 to Justus discloses a method and apparatus for roll profile measurement which also employs piezoelectric sensors.
PCT Application PCT/US96/08204 (International Publication WO 96/38718), assigned to the applicant, discloses a nip pressure and nip width sensing system, certain embodiments of which are directed to measuring nip width using pressure sensitive pads or lines.